In the United States, more than 83 million adults live with cardiovascular diseases making them highly susceptible to heart attack and stroke. 33 percent of all deaths in the U.S. are caused by heart diseases and stroke. In many of these cases the underlying cause is arterial thrombosis, in which platelets play a critical role. Plateles are essential to thrombosis and hemostasis. Upon damage of a blood vessel, circulating platelets rapidly adhere to the site of injury, become activated, and aggregate to form a primary thrombus. The initial platelet adhesion to subendothelial matrix proteins, particularly at high shear rate conditions, is mediated by the interaction between von Willebrand factor (VWF) and its receptor, the platelet glycoprotein (GP) Ib-IX-V complex (GPIb). The interaction of VWF and GPIb is not only important in initiating platelet adhesion, but also for transmitting signals important for full platelet activation. Numerous reports have implicated GPIb in promoting thrombin induced platelet activation. However, it is still unclear how thrombin binding to GPIb is regulated and how thrombin binding to GPIb promotes platelet activation. Using cells genetically engineered to express a truncated form of GPIb? and GPIb? knockout mice, we show that the interaction of GPIb with the intracellular signaling protein 14-3-3 is required for the stimulatory role of GPIb in thrombin-induced platelet activation. We believe that the binding of 14-3-3 to GPIb is required for platelets to become activated in response to low doses of thrombin. Using molecular biology in combination with bone marrow transplantation methods we will test whether deletion of this interaction prevents thrombin-induced platelet responses in mice expressing GPIb with a mutation in a critical 14-3-3 binding site. We hypothesize that the binding of 14-3-3 to GPIb plays an important stimulatory role in thrombin-induced platelet activation. The specific aims of this proposal are: 1) to determine the functional role of the 14-3 3-GPIb interaction in thrombin-induced platelet activation using platelets expressing 14-3-3 binding deficient GPIb and 2) to investigate the mechanism by which GPIb promotes the platelet response to thrombin. Ultimately understanding how thrombin signaling is regulated by the 14-3-3-GPIb interaction will lead to the development of novel drugs for the treatment of patients suffering from heart attack and stroke.